You can rip back to the error of course but it sounds like the mistake is in the initial rows. If you haven't gone too far, you could do that. If you're well along in the shawl and don't want to rip back, you could put a thinner needle into the shawl at the first correct stockinette st row and cut off the mistaken rows. Then you would pick up holding needle and using the correct size needle, knit a new border in garter st (all knit rows).
As an alternaitive, if it wouldn't spoil the line of the shawl, you could just pick up sts in your cast on row and knit the garter st rows. That way you would avoid having to cut off the stockinette rows that are in error.